Implantable cardiac devices are well known in the art. They may take the form of an implantable defibrillator (ICD) to treat accelerated rhythms of the heart such as fibrillation, or an implantable pacemaker to maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
The devices are generally implanted in an upper portion of the left-side of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode-carrying leads which are implanted within the heart. The electrodes are positioned within the heart, for making electrical contact with their designated heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to deliver the desired therapy.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
For defibrillation, one lead may include at least one defibrillation electrode arranged to be positioned in the right ventricle. When fibrillation is detected, a pulse generator delivers a defibrillating shock from the defibrillation electrode in the right ventricle to the device conductive housing to terminate the arrhythmia. Alternatively, a further defibrillation electrode may be positioned in the right atrium or superior vena cava and electrically connected to the right ventricular defibrillation electrode. In this arrangement, the defibrillating shock is delivered from the parallel connected defibrillation electrodes to the conductive housing.
Congestive heart failure (CHF) is a debilitating, end-stage disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body's tissues. As CHF progresses, blood pressure increases and interstitial fluid accumulates in the lungs around the heart. The accumulated fluid fills the gas air exchange space in the lungs and prevents proper lung function. Reduced oxygen saturation further aggravates cardiac function with possible infarction. Hence, CHF requires constant monitoring.
Sleep apnea is another condition which may benefit from constant or frequent monitoring. Sleep apnea is a serious, potentially life-threatening condition characterized by brief interruptions of breathing during sleep. In a given night, the number of involuntary pauses in breathing (apneic events) may be as high as twenty to sixty or more per hour. During sleep apnea, blood oxygen saturation levels are reduced which may be especially serious for a patient with CHF.
As is known, CHF disease state may be evaluated through impedance measurements utilizing electrodes implanted in the heart. Such measurements may be carried out by applying a current between a pair of the electrodes and measuring the voltage between those electrodes or another pair of electrodes. Hence, an implanted cardiac stimulation device is well suited for such an application. Sleep apnea may also be monitored in this manner.
Implantable cardiac devices are also well suited for providing sleep apnea therapy. One such therapy is phrenic nerve stimulation (PNS). Here, stimulation pulses from the device's pulse generator are applied to phrenic nerves associated with the diaphragm or to diaphragm muscle itself. Both of these forms of stimulation therapy are included herein as PNS.
Another form of therapy which an implantable cardiac device is well suited to provide is overdrive pacing. Here, stimulation pulses are provided to the heart to increase the cardiac rate and cardiac output. The stimulation pulses may be in accordance with a pacing modality referred to as DAO pacing where both the atrial and ventricles are paced. The atrial pacing rate is above a base rate and a ventricular pacing pulse is provided an escape interval after each atrial pacing pulse. DAO pacing is considered effective at preventing central sleep apnea because the higher cardiac rate will increase cardiac output which in turn will decrease the delay in the respiratory control loop.
DAO pacing will achieve its intended purpose only when a patient's blood saturation is not deteriorated. Without proper ventilation, DAO pacing may actually be deleterious because the heart muscle may be forced by the pacing to consume enough oxygen to diminish oxygen supply to the myocardium. If such a condition were to continue, a myocardial infarction could result which in turn could result in a heart attack. The present invention addresses this and other issues which shall become apparent.